Reproducible builds are like making a cake from a recipe. If you have a good recipe, you can use the same ingredients and instructions to make the same cake every time. Similarly, in computer programs, a reproducible build means that if someone else takes the same source code and builds it, they will get the same executable program every time.
This is really important because it allows people to verify that the program they get is the exact same one that the developer released. This can help protect against security vulnerabilities or software bugs.
Think of it this way, if you were a chef and you only had one chance to make a cake for a competition, you would want to be sure that your recipe worked every time. The same is true for software development, if someone is putting their program out into the world, they want to be sure that it will work as intended every time someone else builds it from the source code.